Conventional communications makes widespread use of spread spectrum techniques. Typically, each of these spread spectrum techniques has a mechanism for spreading baseband or message information over a much wider spectrum than is required to transmit the information. In the art, the term spread spectrum is understood as spectrum spreading in a manner that is independent of the content of the information being spread. This definition rules out such techniques as wideband FM for inclusion under the spread spectrum rubric.
Spread spectrum communication techniques have several advantages. First, by spreading the information over a much wider bandwidth than needed, it is possible to insinuate a certain degree of coverture in the communications and thus making it more difficult in general to detect the communications. Typically, this feature is of use in promoting communications security. A second advantage is that the spreading of information can be performed in a manner that is unpredictable by a third party and thus making it more difficult, in general, to defeat or jam the communications. Similarly, this feature promotes communications security. A third advantage is spectrum sharing. This feature allows a plurality of users to use the same spectrum at the same time. One disadvantage of this feature is a degradation in quality for any particular link as more simultaneous transmissions are conducted. The spectrum sharing feature is of interest to current commercial communications.
Typically, spread spectrum techniques can be categorized into at least three distinct classes that can be practiced individually or in combination. These classes include frequency hopping, time hopping, and direct sequence. Frequency hopping techniques modulate the baseband to different center frequencies within the shared spectrum space. Time hopping techniques encompass low duty cycle bursting of information. Direct sequence techniques are generally practiced by modulo-two adding a high-speed pseudorandom or noise-like binary spreading sequence to the digitized baseband information. This results in the modulo-two bit sum bit sequence having a bandwidth that is as wide as the bandwidth of the pseudorandom or noise-like spreading sequence.
Direct sequence spread spectrum techniques can be practiced using one of at least two techniques. The first technique is to generate the same pseudorandom or noise-like spreading sequence at both the transmitter and receiver with relative delay in order to match the propagation delay between transmitter and receiver. This technique requires that synchronization be established and maintained between transmitter and receiver at a precision dictated by the rate of the pseudorandom or noise-like sequence. Effecting this technique can be a significant technical challenge. The second technique to practice direct sequence spread spectrum is to transmit the pseudorandom or noise-like spreading sequence in addition to transmitting the modulo-two bit sum bit sequence. Depending upon its implementation, this second technique can require more power or more bandwidth than the first way, but an advantage of this second technique is that synchronization is generally less difficult.
In spread spectrum communications systems, in particular time-delayed transmitted reference spread spectrum communication systems, it is necessary to generate a wideband noise-like carrier and its time-delayed replica. There is a further desire to provide spectral shaping to the wideband carrier in order to avoid transmitting significant energy within spectrum reserved and protected for other communication users. Further, it would be desirable to generate the wideband carrier and its time-delayed replica deterministically by a finite state digital machine capable of running at a very high speed.